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Desliming and Drain & Rinse Screens

An Introduction
to Desliming and Drain & Rinse Applications

In mineral processing two common screen applications are
desliming and dense media drain and rinse.Sizing the screen area is mostly by size distribution and
feed rates, the addition of wash water is also very important.This discussion is about determining the amount of water
needed for these two purposes.

DESLIMING

In mineral processing desliming refers to the removal of very fine
particles from a feed stream that can cause operational difficulties
in the next and latter processing steps.For flotation it is often done at around 50 microns.For dense media operations it is normally done at 0.5 mm (500
microns or 28 mesh(nominal)).The following discussion is about the second operation,
desliming at 0.5 mm.

For Dense Media applications, desliming is normally defined
as the removal of the -0.5 mm fraction from the feed.This is particularly important when feeding material that is
finer than 10 mm.
If the amount of -0.5 mm in the -10 mm product is less than or equal
to 6% by weight, desliming is not necessary, but may improve
operations.If the
amount of -0.5 mm in the -10 mm product (for industrial minerals,
ores, and coal) is equal to or less than 10% but more than 6%
desliming by vibrating screens only is possible.If the amount of -0.5 mm in the -10 mm product (for
industrial minerals, ores, and coal) is more than 10% then sieve
bends followed by screens is recommended.

The normal liquid used is water, except in cases where the
treated mineral is water soluble than a saturated brine solution
should be used.Since
in most cases water is the liquid that is used, thatterm will used for this discussion.

The amount of water used for desliming should give a solids
concentration of 7% by volume in the underflow from the desliming
stage.

Where:

A = the amount of water required

a = % by weight of -0.5 mm in the feed -10 mm

S = amount of -10 mm in the feed in t/hr

= the specific gravity
of solids in the feed (average)

The
+0.5 mm material in the feed has no effect on the total amount of
desliming water required.In general, at least 8 m3/hr of water per 1 m of screen width
should be used.

Two methods of adding the wash water are generally used,
sprays and overflow boxes (weirs).

For sprays either nozzles or “fish tails” are used.When using nozzles there should be a minimum of 3 meters of
head on the sprays.And the general design is 4 nozzles per meter of screen
width.For “fish tail”
spray bars, the general design is to use 2 sprays for each meter (3
foot) of screen width.

DRAIN AND RINSE

The float and sink products from a dense media process are processed
in drain and rinse circuits that normally contain a sieve bend
followed by a vibrating screen.Alternatively two sieve bends followed by a screen or a large
banana screen can be used.The most common has been the sieve bend followed by a
vibrating screen.

80% to 85% of the media is removed by the sieve bend.The first part of the screen continues the draining with the
sieve bend and drain section media going back to the correct media
sump for immediate reuse.A common practice is to take a minor split (less than or
equal to 10%) of this stream to the media recovery circuit to
minimize build-up of fine solids in the media.The drain portion is normally about 25% of the screen length.

The second part of the screen is normally equipped with sprays or
wash water to remove any remaining media adhering to the particles.This goes to a media recovery circuit to reclaim the media.Rinse is normally done in two sections, with the first using
recycled water (if available) and being about 75% of the flow, and
the second (final) portion being fresh water and 25% of the flow.

Following the drain sectionthe amount of media still adhering to the
material can generally be calculated by: